Syria's government and opposition, meeting for the first time, have vented their mutual hostility but a UN mediator says the warring sides may be ready to discuss prisoner swaps, local ceasefires and humanitarian aid.

The talks in Montreux on Lake Geneva got off to a bad start with the Syrian government accusing the opposition of being terrorists, whose activities are being supported by neighbouring countries.

The United Nations secretary-general got into an argument with Syrian foreign minister Walid Muaellem when he overran his allocated time to speak.

When Mr Ban tried to intervene to get him to "wrap up", Mr Muaellem told the head of the UN that he had the right to speak, pointing out that he lived in Syria whereas Mr Ban lived in New York.

Opposition wants power transferred from Assad

Mr Jarba has called for the government delegation to turn against their president before direct negotiations between the opposition and the government start on Friday.

"We want to make sure we have a partner in this room that goes from being a Bashar al-Assad delegation to a free delegation so that all executive powers are transferred from Bashar al-Assad," he said.

"My question is clear: Do we have such a partner?"

But Mr Muaellem insists Mr Assad's future is not up for discussion.

"We came here as representatives of the Syrian people and state and everybody should know that nobody in this world has the right to withdraw the legitimacy of a president or government ... other than the Syrians themselves," he said.

Allegations of torture

He fired a broadside at the reform-shy Gulf monarchies which back the revolt against Mr Assad's regime.

"It is regrettable to me and to the people of Syria that representatives of states in this room are sitting with us today, while blood is on their hands - countries that have sent weapons... encouraged and financed terrorism," he said.

"They have not looked at their own glass houses before throwing stones."

Russia says the rival sides have promised to start direct talks despite fears that a standoff over Mr Assad's fate at the meeting would halt the push for a political solution to Syria's civil war, which has killed over 130,000 and made millions homeless.

Even if the sides are willing to talk about limited confidence-building measures, expectations for the peace process remain low, with an overall solution to the three-year war still far off.

International mediator Lakhdar Brahimi has signalled that both sides are ready to move beyond rhetoric.

"We have had some fairly clear indications that the parties are willing to discuss issues of access to needy people, the liberation of prisoners and local ceasefires," he told a news conference.

He will meet both Syrian delegations separately on Thursday (local time), before trying to bring them into the same room later in the week.

"We will try to see if we meet Friday morning separately and hopefully by Friday afternoon both sides will sit in one room," he said.

"We have no illusion that it is going to be easy but we are going to try very hard."